Weak roots in the derived forms (أراد، اختار، اشترى)

Weak roots in the derived forms

You have used أراد, اختار, اشترى and انتهى since your first lessons. They are derived forms built on a weak root: one with و or ي in the middle (a hollow root) or at the end (a defective root). In a hollow root that letter becomes a long vowel in Forms IV, VII, VIII and X (in Forms II, III, V and VI it stays a consonant: غيّر، عاون); in a defective root it becomes ى or ي. Either way, it changes shape as the verb changes.

Hollow roots: the long vowel shortens

The long vowel shortens before an ending that starts with a consonant (أردت، اخترنا) and after لم when nothing follows (لم يرد, but لم يريدوا). In the present, Forms IV and X have ي; Forms VII and VIII keep ا: انهار ينهار, ارتاح(irtâḥ) يرتاح.

Careful: written without vowels, لم يرد can also be لَمْ يَرُدَّ did not reply. The sentence decides.

Defective roots: ى becomes ي

Before a consonant ending, ى becomes ي: اشتريت، انتهينا. The present ends in ي, except in Forms V and VI, which keep ى (تفادى يتفادى). After لم the final vowel drops: لم يشتر, and اكتفى ب(iktafâ bi) gives لم يكتف.